This invention relates generally to flexible diaphragms of the type that serve as flow control valves in respirators or volume ventilators. More particularly, the invention relates to diaphragms for use in exhalation valves of respirators.
A respirator uses a system of tubes to direct a flow of gases to and from a patient. Typically, in a patient breathing circuit there are separate tubes for inhalation and exhalation, the two tubes merging into a common duct near the patient. Valves in the inhalation and exhalation tubes open and close at appropriate times to regulate the breathing cycle. For example, an exhalation valve in the exhalation tube is allowed to open as the patient breathes out, while a check valve simultaneously closes the inhalation tube to prevent flow of exhaled gas into the inhalation tube.
An exhalation valve commonly used in respirators includes a resilient diaphragm mounted in a valve assembly having an inlet port, an outlet port and a control pressure port. In a closed position, the diaphragm rests on a valve seat and covers the inlet port. In the open position, the diaphragm lifts from the valve seat and places the inlet port in fluid communication with the outlet port. The pressure control port admits a control pressure to be applied to the diaphragm from above, and the valve is firmly closed when a relatively large control pressure is applied to the diaphragm. The valve opens when the control pressure is reduced sufficiently to be overcome by expiration pressure supplied by the patient.
This type of valve works satisfactorily for moderate or high flows of exhaled gas, but has a significant drawback at low exhalation flow rates, such as are encountered at the very end of the exhalation phase of a breathing cycle. The problem is particularly acute when the intention is to close the exhalation valve while there is still a small positive pressure within the patient's lungs. This is accomplished by maintaining a small positive control pressure on the diaphragm during the exhalation phase. As the patient exhales, the pressure on the lower face of the diaphragm becomes gradually less, until it is ultimately exceeded by the control pressure on the upper face of the diaphragm. Then the diaphragm should close and prevent any further exhalation flow.
What happens in practice, however, is that at low flows the diaphragm will close initially, and then open again when the exhalation pressure builds up slightly, then close again and open again in a cyclic pattern. This hunting movement of the diaphragm produces undesirable vibration and noise that can be propagated through the entire patient breathing circuit. Accordingly, there has been a significant need for an exhalation valve which substantially reduces vibration and noise under low exhalation flow conditions. The present invention satisfies this need.